65489 Ceto/ˈsiːtoʊ/ is a binarytrans-Neptunian object (TNO) discovered on March 22, 2003 by C. A. Trujillo and M. Brown at Palomar. It is named after the sea goddess Ceto from Greek mythology. The object was identified as a binary on April 11, 2006 by K. Noll, H. Levison, W. Grundy and D. Stephens using the Hubble Space Telescope; the companion object is named Phorcys (/ˈfɔərsɨs/, formally (65849) Ceto I Phorcys), after the Greek sea god. The Ceto system is considered the second known binary centaur,[3] using an extended definition of a centaur as an object on a non-resonant (unstable) orbit with the perihelion inside the orbit of Neptune.[5] It came to perihelion in 1989.[4]

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Physical characteristics[edit]

65489 Ceto is an example of a close binary TNO system in which the components are of similar size. Combined observations with the infrared Spitzer Space Telescope and the Hubble Telescope allow the diameter of Ceto itself to be estimated at 174+16 −18 km and the diameter of Phorcys at 132+6 −14 km, assuming equal albedo for both components.[3]

The binary nature of Ceto enables direct calculation of the system mass, allowing estimation of the masses of the components and providing additional constraints on their composition. The estimated density of Ceto is 1.37+0.66 −0.32 g/cm3, significantly less than that of the large TNOs (Haumea: 3.0 g/cm3, Eris: 2.26, Pluto: 2.03, Charon: 1.65) but significantly more than that of smaller TNOs (e.g. 0.7 g/cm3 for (26308) 1998 SM165). Phorcys has a mass of about 1.67×1018 kg.[3] Unless the bodies are porous, the density is consistent with rock–ice composition, with rock content around 50%.[3]

It has been suggested that tidal forces, together with other potential heat sources (e.g. collisions or 26Aldecay) might have raised the temperature sufficiently to crystallise amorphous ice and reduce the void space inside the object. The same tidal forces could be responsible for the quasi-circular orbits of the components of Ceto.[3]